Derivatives of 1-hydroxymethylene-1,1-bisphosphonic acid display remarkable antitumoral properties and their medical applications have been therefore the subject of in-depth research. Such derivatives, characterized by a P—C—P bond, are stable analogs of pyrophosphate and are resistant to enzymatic hydrolysis. Regarding the therapeutic applications of diphosphonic acid derivatives, it is well known that various derivatives have properties useful in the treatment of inflammation, osteoporosis or some bone metastasis. In particular, they are used for their ability to inhibit bone resorption to treat numerous diseases characterized by abnormal calcium metabolism. Bone resorption is pathologically increased in metastasis of certain types of cancer such as breast or prostate cancer, and is accelerated in different forms of osteoporosis including that related to age. Patients with this type of metastasis can now benefit from treatment regimens including bisphosphonates (Diel et al., 2000; Lipton, 2000; Mincey et al., 2000). It should be pointed out that bone is the third most common site of metastasis and that over 80% of patients who die from cancer have bone tumors at autopsy.
A number of diphosphonic acid derivatives and their properties useful in various applications have been described in the literature.
For example, didronic acid has been known for years as active for drugs in the treatment of bone diseases such as osteoporosis, and more particularly disodium didronate described in French patent 8,441 M. A derivative is described for example in U.S. Pat. No. 4,705,561 relating to alendronic acid which inhibits bone resorption and which can be used in the treatment of osteoporosis. Another similar structure is described in British patent 2,312,165 relating to ibandronic acid having anti-inflammatory properties.
Alkane-1,1-diphosphonic acid with an amino-acid, and possessing antitumor and bone resorption activities have been described in PCT patent application WO 97/49711. Other diphosphonic acid derivatives comprising a phenyl substituent at the 1 position are described in U.S. Pat. No. 4,473,560 which discloses their anti-inflammatory activity, more particularly a antiarthritic activity, or in PCT patent application WO 97/04785 relating to phenol substituted diphosphonates having antineoplasic activity. Further, European patent EP 537,008 describes diphosphonate derivatives having a lipophilic group, which are useful in the preparation of a medicament inhibiting protein prenyl transferase, likely to block the neoplasic transformation resulting from ras oncogenes.
This anti-osteoclastic action of bisphosphonates is postulated to occur by induction of apoptosis in osteoclasts (Luckman et al., 1998) via inhibition of the mevalonate pathway and of cholesterol synthesis.
Bisphosphonates inhibit in vitro the proliferation of breast tumor cells (Fromigue et al., 2000; Hiraga et al., 2001; Jagdev et al., 2001; Senaratne et al., 2000; Yoneda et al., 2000) and prostate tumor cells (Lee et al., 2001). This in vitro inhibition is due to apoptosis of tumor cells and is accompanied by expression of the bcl-2 gene (Senaratne et al., 2000) and activation of caspases (Fromigue et al., 2000).
Besides the hereinabove effects, bisphosphonates may have several additional actions, such as inhibition in vitro of adhesion of breast tumor cells to bone matrices (Boissier et al., 1997; Van der Pluijm et al., 1996), induction in vitro of myeloma cell apoptosis (Shipman et al., 1997, 1998, 2000a) or inhibition of the activity (but not the production) of matrix metalloproteinases in breast or prostate carcinoma cells (Boissier et al., 2000; Ichinose et al., 2000; Teronen et al., 1997).
Bisphosphonates have also been utilized in the treatment of lymphoblastic leukemia (Ogihara et al., 1995; Takagi et al., 1998). Leukemic cells induce angiogenesis in bone marrow, this being necessary for their proliferation. Treatment of lymphoblastic leukemia with bisphosphonates is accompanied by a significant decrease in angiogenesis (Perez-Atayde et al., 1997).
Recent data point toward the very likely involvement of angiogenic factors in the formation of bone metastases. For instance, in a study in an in vivo murine model of experimental breast tumor cell metastasis (MDA MB 231), Van der Pluijm et al. (2001) hypothesized that elevated expression of angiogenic (VEGF) and osteolytic (PTH) factors in the tumor cells is involved in osteotropism and bone loss of bone metastases.
Although the antiproliferative activity of bisphosphonates on tumor cells in vitro is now well established, the question of whether bisphosphonates can exert antitumoral action in vivo is still open. Some data appear to argue for an antiproliferative action of bisphosphonates in vivo, at metastatic sites in bone. For instance, according to Hiraga et al. (2001), bisphosphonates would diminish the tumor burden of metastatic cells in bone. However, to our knowledge, no data have been reported on an antitumoral action of bisphosphonates in vivo on primary tumors.
Bisphosphonates are metabolized by the body to a low extent and the active fraction represents only 3 to 7% of the absorbed dose. This low bioavailability of bisphosphonates after oral administration results from their low lipophilicity (Lin, 1996) which is due to their high state of ionization at physiologic pH. Their absorption is further reduced by the strong negative charge and fairly large size of these molecules (Ruifrok and Mol, 1983; Pade and Stavchanvsky, 1997). Moreover, the absorption of bisphosphonates is reduced further still by their high level of complexation with calcium and other divalent ions in the intestine (Lin, 1996). Their administration often causes gastrointestinal symptoms and other side effects (Adami and Zamberlan., 1996; Mondelo et al., 1997).
Moreover, drug acquired resistance is now a major concern in the cancer therapy, and most of human cancers are resistant to the effects of chemiotherapies.
To improve their therapeutic effects, various approaches have been proposed. The first consists in the use of a peptide vector grafted to the side chain of hydroxybisphosphonic acid (Ezra et al., 2000). Other studies suggest encapsulating the drug in microspheres (Patashnik et al., 1997) or in liposomes (Ylitalo et al., 1998).
The present invention is therefore directed at providing new bisphosphonate derivatives with improved bioavailability and satisfactory therapeutic efficacy.
Two methods of synthesis described in the literature yield 1-hydroxymethylene-1,1-bisphosphonic acids.
In the first, the desired products are obtained in a single step. This method consists in heating a mixture of carboxylic acid in the presence of phosphorous acid and phosphorus trichloride at 100° C. for several hours.
The conditions of this reaction have been studied in great detail. In fact, since 1970, more than fifty patents and articles have been published. The drawbacks of this method, however, are numerous. Indeed, the drastic operating conditions are not suited to labile substrates. Furthermore, extraction of the bisphosphonate from the reaction medium is often tricky.
The second procedure is an indirect method involving synthesis of 1-hydroxymethylene-1,1-bisphosphonic esters followed by a dealkylation step. The α-ketophosphonates are typically prepared via a Michaelis Arbuzov reaction, starting from a phosphite with the structure P(OR)3 and an acid chloride. The bisphosphonic ester is then normally obtained by reaction of the α-ketophosphonate with a dialkylphosphite HOP(OR)2.
The dealkylation step is carried out either by hydrolysis in hydrochloric acid or by treatment with bromotrimethylsilane followed by methanolysis.
Unfortunately, neither of these two synthetic methods yields partially esterified 1-hydroxymethylene-1,1-bisphosphonic acids.
The present invention therefore equally proposes methods of preparation of bisphosphonates allowing regioselective addition of one or more ester functions on the phosphonic acid moieties.